1. Field of the Invention
The present invention relates to an image forming optical system and an electronic image pickup apparatus equipped with the same.
2. Description of the Related Art
With the progress in the slimming of compact cameras in recent years, slimming of optical systems are required so that they can be accommodated in slim camera bodies. The optical system of compact cameras is collapsed and stored in the body after use. For this reason, the slimming of the camera body necessitates the slimming of the collapsed length of the optical system accordingly.
To enable the collapsing of an optical system, the lens frame of the optical system is divided into a plurality of tiers so that it can extend and collapse. In this case, if the collapsed length is to be made small, the length of each divisional tier of the lens frame is also to be made small. This leads to an increase in the number of tiers for collapsing. In consequence, the entire lens frame tends to be deformed by gravity when the optical system is extended, and decentering of the optical system tends to occur. In view of this, it is necessary to reduce the entire length of the optical system to thereby shorten the lengths of divisions of the lens frame and reduce the number of divisions.
To provide an optical system that meets the above requirements, it is particularly important to slim the optical system in the collapsed state and to reduce the entire length. Structures that focus on the slimming are described, for example, in Japanese Patent Application Laid-Open No. 11-258507 and Japanese Patent Application Laid-Open No. 2007-72117.
In the fourth, sixth, and seventh embodiments disclosed in Japanese Patent Application Laid-Open No. 11-258507, the first lens group is composed of a single lens element. In general, in particular a reduction in the entire length of an optical system leads to an increase in the refracting power of the first lens group. This causes large chromatic aberrations such as axial chromatic aberration and chromatic aberration of magnification, leading to a deterioration in the image quality.
To suppress chromatic aberrations, at least two lenses (i.e. a positive lens and a negative lens) need to be used. However, if two lenses are used, the thickness of the lens system will become thicker than in the case of a single lens system. In view of this, the two lenses are cemented together in many cases so as to make the thickness as small as possible.
Such an optical system is disclosed, for example, in Japanese Patent Application Laid-Open No. 2007-72117. In this optical system, the first lens group is composed of a cemented lens made up of a positive lens and a negative lens arranged in order from the object side. Conventionally, cementing of the cemented lens is performed after machining each of the lens elements. Therefore, the two lenses in such a cemented lens each need to have a certain degree of thickness to allow machining. For this reason, the thickness of the cemented lens and the thickness of the optical system in the collapsed state tend to become thick.
Furthermore, such cemented lenses tend to be heavy in weight. The amount of movement of the first lens group in optical systems having a variable entire length is generally large. The first lens group that is heavy in weight necessitates a large drive system (such as a motor) and a large overall size of the camera.